fdt: Makefile: Ensure that OF_BOARD is used when needed

Boards which define OF_HAS_PRIOR_STAGE must define OF_BOARD at present,
since a custom function is the only way to obtain the devicetree at
runtime.

Add a build error when this requirement is not met, to avoid accepting
any patches which break this requirement.

Add an allowlist for boards which use it, currently none. This allowlist
can be updated for local development, if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: change of_whitelist to of_allowlist]
Signed-off-by: Tom Rini <trini@konsulko.com>